Process for the preparation of vanillin

ABSTRACT

The present invention relates to a process for preparing vanillin, comprising an electrolysis of an aqueous, alkaline lignin-comprising suspension or solution, wherein, as anode material, a base alloy is used which is selected among Co-base alloys, Fe-base alloys, Cu-base alloys and Ni-base alloys.

The invention relates to a process for preparing vanillin which comprises an electrolysis of an aqueous, alkaline lignin-comprising suspension or solution.

The transformation of renewable raw materials to valuable chemicals which are suitable, particularly, as fragrance and aroma substances, is of very great interest. Lignin and also lignin-comprising substances such as alkali lignin, lignin sulfate or lignin sulfonate arise as waste materials or byproducts of wood processing to give pulp. The total production of lignin-comprising substances is estimated at about 20 billion metric tonnes per year. Lignin is thus a valuable raw material. Parts of this lignin are in the interim further used. For example, alkali lignin, which can be produced by alkaline treatment of the black liquor arising in paper manufacture, is used in North America as a binder for particle boards based on wood and cellulose, as dispersants, for clarifying sugar solutions, stabilizing asphalt emulsions and also foam stabilization. However, by far the greatest amount of waste lignin is used via combustion as an energy source, e.g. for the pulp process.

The biopolymer lignin is a group of three-dimensional macromolecules occurring in the cell wall of plants that are composed of various phenolic monomer building blocks such as p-cumaryl alcohol, coniferyl alcohol and sinapyl alcohol. On account of its composition, it is the sole significant source of aromatics in nature. The use of this renewable natural material, in addition, does not compete with a use as food.

Vanillin, 4-hydroxy-3-methoxybenzaldehyde, is a synthetic aroma substance which is widely used instead of expensive natural vanilla as an aroma substance for foods, as a fragrance in deodorants and perfumes, and also for flavor enhancement of pharmaceuticals and vitamin preparations. Vanillin is also an intermediate in the synthesis of various medicaments such as e.g. L-dopa, methyldopa and papaverine.

To date, aromatic aldehydes have generally been produced from petrochemical precursors. On account of the structural similarity of vanillin to the building blocks of lignin, lignin should also be suitable as a starting material for producing vanillin. The oxidative cleavage of lignin to vanillin and other aromatic aldehydes has therefore been the subject of numerous studies since the 1940s. The most frequently used conversions of lignin are chemical oxidation with copper oxide (see J. M. Pepper, B. W. Casselman, J. C. Karapally, Can. J. Chem. 1967, 45, 3009-3012) or nitrobenzene (see B. Leopold, Acta. Chem. Scand. 1950, 4, 1523-1537; B. Leopold, Acta. Chem. Scand. 1952, 6, 38-39, acidolysis (see J. M. Pepper, P. E. T. Baylis, E. Adler, Can. J. Chem. 1959, 37, 1241-1248), hydrogenolysis (see F. E. Brauns, Academic Press 1952, New York, 511-535) or ozonolysis (C. Doree, M. Cunningham, J. Chem. Soc. 1913, 103, 677-686). One of the leading methods is treating lignin with oxygen at 150° C. in an alkaline medium in the presence of copper or cobalt catalysts (see H. R. Bjorsvik, Org. Proc. Res. Dev. 1999, 3, 330-340).

WO 87/03014 describes a process for the electrochemical oxidation of lignin at temperatures of preferably 170 to 190° C. in aqueous, strongly alkaline solutions. As anodes, primarily electrodes made of copper or nickel are used. As low-molecular-weight product, a complex mixture is obtained which, inter alia, comprises vanillin acid (4-hydroxy-3-methoxybenzoic acid), vanillin, 4-hydroxybenzaldehyde, 4-hydroxyacetophenone and acetovanillone (4-hydroxy-3-methoxyacetophenone) and also optionally phenol, syringaic acid (4-hydroxy-3,5-dimethoxybenzoic acid) and syringaldehyde (4-hydroxy-3,5-dimethoxybenzaldehyde). Generally, 4-hydroxybenzoic acid is the main product. The selectivity with respect to the vanillin formation is low and only at high temperatures is it reasonably satisfactory. In addition, under the comparatively drastic reaction conditions, severe corrosion of the electrode materials takes place. This corrosion is also a problem with respect to contamination of the vanillin with heavy metals. Furthermore, the high temperatures are unfavorable from the energetic point of view. A temperature reduction, however, leads to a significant loss in selectivity.

C. Z. Smith et al., J. Appl. Electrochem. 2011, DOI 10.1007/s10800-010-0245-0 likewise describes investigations on the electrochemical oxidation of lignin sulfate to vanillin under alkaline conditions at nickel electrodes at temperatures of 170° C. As electrolysis cell, a cell having circulation is used, in which the lignin sulfate-comprising electrolyte is circulated continuously through a cylindrical electrode arrangement having a central cylindrical nickel grid as cathode and a nickel grid cylindrically surrounding the cathode as anode. The problems of selectivity and corrosion problems are not solved thereby.

WO 2009/138368 describes a process for the electrolytic breakdown of lignin, in which an aqueous lignin-comprising electrolyte is oxidized at a diamond electrode. In this process, inter alia, a low-molecular-weight product is formed that comprises, roughly in equal fractions, vanillin together with other hydroxybenzaldehyde derivatives such as acetovanillone or guaiacol. The selectivity of the lignin oxidation with respect to vanillin is low. As the inventors' own investigations show, under the reaction conditions, corrosion of the diamond electrode takes place.

The earlier European patent application 11177320.6 describes, for production of vanillin, the electrolysis of lignin-comprising solutions or suspensions, wherein, as anode material, silver or silver-comprising alloys are used.

An object of the present invention is provision of a process which permits the production of vanillin from lignin or lignin-comprising substances in good yields and with high selectivity with respect to vanillin formation. In addition, the process should be able to be carried out under milder conditions than the processes of the prior art. It is also an object to improve problems of corrosion. In particular, the vanillin should be obtained in a form which does not exclude its use as an aroma substance.

These and further objects are achieved by the process described hereinafter, in which an aqueous, alkaline lignin-comprising suspension or solution is electrolyzed, wherein, as anode material, a base alloy is used which is selected among Co-base alloys, Fe-base alloys, Cu-base alloys and Ni-base alloys.

The present invention therefore relates to a process for preparing vanillin, comprising an electrolysis of an aqueous, alkaline lignin-comprising suspension or solution, wherein, as anode material, a base alloy is used which is selected among Co-base alloys, Fe-base alloys, Cu-base alloys and Ni-base alloys.

The process according to the invention is linked to a number of advantages. Thus, the electrode materials used lead to a significant increase in selectivity. This high selectivity can surprisingly be achieved even at a comparatively low temperature of up to 100° C. In addition, the anode materials used according to the invention prove to be extremely resistant to the corrosive reaction conditions and, in contrast to the processes of the prior art, no corrosion or no significant corrosion takes place.

In the process according to the invention, an aqueous, lignin-comprising electrolyte which comprises lignin or a lignin-comprising substance and is present in the form of an aqueous suspension or solution, is subjected to an electrolysis under alkaline conditions. In this case, the oxidation of the lignin present or of the lignin derivative takes place at the anode. At the cathode, typically, reduction of the aqueous electrolyte proceeds, e.g. with formation of hydrogen.

In the process according to the invention, one or more anodes made of a base alloy are used as anode material, wherein the base alloy is selected among Co-base alloys, Fe-base alloys, Cu-base alloys and Ni-base alloys.

A base alloy is taken to mean an alloy which comprises at least 50% by weight, in particular at least 55% by weight, especially at least 58% by weight, e.g. 50 to 99% by weight, preferably 50 to 95% by weight, in particular 55 to 95% by weight, particularly preferably 55 to 90% by weight, and especially 58 to 90% by weight, of the respective base metal (in the case of a Co-base alloy Co, in the case of a Cu-base alloy Cu, in the case of an Ni-base alloy Ni, and in the case of an Fe-base alloy Fe) and at least one further alloy component, wherein the total amount of all further alloy components different from the base metal is typically at least 1% by weight, in particular at least 5% by weight, and especially at least 10% by weight, and is, e.g. in the range from 1 to 50% by weight, preferably in the range from 5 to 50% by weight, in particular in the range from 5 to 45% by weight, particularly preferably in the range from 10 to 45% by weight, and especially in the range from 10 to 42% by weight, wherein all figures in percent by weight are in each case based on the total weight of the alloy.

Typical further alloy components are especially Cu, Fe, Co, Ni, Mn, Cr, Mo, V, Nb, Ti, Ag, Pb and Zn, and also Si, C, P and S. Preference is accordingly given to base alloys which comprise at least one further alloy component among the abovementioned alloy components different from the base metal.

Preference, in particular with respect to stability thereof with simultaneously good selectivity and/or good yield, is given to Ni-base alloys, Fe-base alloys and Co-base alloys, in particular Ni-base alloys and Co-base alloys.

Preference, in particular with respect to selectivity thereof with simultaneously satisfactory stability, is given to Cu-base alloys.

Accordingly, a first embodiment of the invention relates to a process in which the anode material is an Ni-base alloy. Typical nickel-base alloys comprise substantially, i.e. at least 95% by weight and in particular at least 98% by weight, and especially at least 99% by weight of

-   -   a1) 50 to 95% by weight, in particular 55 to 95% by weight,         particularly preferably 55 to 90% by weight, and especially 58         to 90% by weight, of Ni and     -   b1) 5 to 50% by weight, in particular 5 to 45% by weight,         particularly preferably 10 to 45% by weight, and especially 10         to 42% by weight, of at least one further alloy component,         selected among Cu, Fe, Co, Mn, Cr, Mo, W, V, Nb, Ti, Si, Al, C         and S.

In the Ni-base alloys, the total amount of Al, Si, C and S will preferably not exceed 5% by weight. Typical quantitative fractions of the further alloy components that can be present in Ni-base alloys in an amount significant for the alloy are stated in the table 1 hereinafter:

TABLE 1 Further alloy components of Ni-base alloys Alloy component Amount [% by weight] Cu   5 to 35 Fe 0.5 to 18 Co   5 to 42 Mn 0.5 to 5 Cr   5 to 40 Mo   5 to 35 W 0.5 to 5 V   1 to 22 Nb   1 to 20 Ti 0.5 to 7 Al 0.1 to 3 Si 0.1 to 3 C 0.1 to 3 S 0.1 to 3

Among the Ni-base alloys of the first embodiment, preference is given in particular to those that comprise 5 to 35% by weight, in particular 10 to 30% by weight, of Cu as further alloy component. These alloys are termed hereinafter group 1.1. In addition to Cu, the base alloys of group 1.1 can comprise one or more of the following alloy components in an amount of up to 45% by weight, in particular up to 40% by weight: Fe, Co, Mn, Cr, Mo, W, V, Nb, Ti, Si, Al, C and S. Preferably, the further alloy component, where present, will be present in an amount stated in table 1.

Examples of Ni-base alloys of group 1.1 are alloys of the EN abbreviations NiCu30Fe (Monel) 400) and NiCu30Al and also the nickel-Cu alloy of the following composition: 63% by weight of Ni, 30% by weight of Cu, 2% by weight of Fe, 1.5% by weight of Mn, 0.5% by weight of Ti (Monel 500K).

Among the Ni-base alloys of the first embodiment, preference is also given to those, in particular, that comprise 5 to 40% by weight, in particular 15 to 30% by weight, of Cr as further alloy component. These alloys are termed hereinafter group 1.2. In addition to Cr, the base alloys of group 1.2 can comprise one or more of the following alloy components in an amount of up to 40% by weight, in particular up to 35% by weight: Fe, Co, Mn, Cu, Mo, W, V, Nb, Ti, Si, Al, C and S. Preferably, the further alloy component, where present, is present in an amount stated in table 1. Among the Ni-base alloys of group 1.2, preference is given, in particular, to those that comprise Mo, Nb and/or Fe as further alloy component, in particular in an amount of in total 1 to 30% by weight.

Examples of Ni-base alloys of group 1.2 are alloys of the EN abbreviations NiCr19NbMo (Inconel® alloy 718) and NiCr15Fe (Inconel® alloy 600), NiCr22Mo19Fe5 (Inconel® 625), NiMo17Cr16FeWMn (Hastelloy® C276), an Ni—Cr—Fe-alloy having a nickel content of 72 to 76% by weight, a Cr content of 18 to 21% by weight, a C content of 0.08 to 0.13% by weight, and an Fe content of 5% by weight, and an Ni—Cr—Co—Mo-alloy having a nickel content of 48 to 60% by weight, a Cr content of 19% by weight, a Co content of 13.5% by weight, and an Mo content of 4.3% by weight (Waspaloy®).

Among the Ni-base alloys of the first embodiment preference is also given, in particular, to those that comprise 5 to 35% by weight, in particular 10 to 30% by weight, of Mo as further alloy component. These alloys are termed hereinafter group 1.3. In addition to Mo, the base alloys of group 1.3 can comprise one or more of the following alloy components in an amount of up to 40% by weight, in particular up to 35% by weight: Fe, Co, Mn, Cu, Cr, W, V, Nb, Ti, Si, Al, C and S. Preferably, the further alloy component, where present, is present in an amount stated in table 1. Among the Ni-base alloys of group 1.3, in particular preference is given to those which comprise Cr, Nb and/or Fe as further alloy component, in particular in an amount of in total 1 to 30% by weight.

Examples of Ni-base alloys of group 1.3 are alloys of the EN abbreviations NiMo28 (Hastelloy® B and Hastelloy® B-2) and NiMo29Cr (Hastelloy® B-3).

Among the Ni-base alloys of the first embodiment, preference is given in particular to those of groups 1.2 and 1.3 preferably with respect to high stability with simultaneously high selectivity.

A second embodiment of the invention relates to a process in which the anode material is a Co base alloy. Typical cobalt base alloys comprise substantially, i.e. at least 95% by weight, in particular at least 98% by weight, and especially at least 99% by weight of:

-   -   a2) 50 to 95% by weight, in particular 55 to 95% by weight,         particularly preferably 55 to 90% by weight, and especially 58         to 90% by weight, of Co and     -   b2) 5 to 50% by weight, in particular 5 to 45% by weight,         particularly preferably 10 to 45% by weight, and especially 10         to 42% by weight, of at least one further alloy component,         selected among Cu, Fe, Ni, Mn, Cr, Mo, W, V, Nb, Ti, Si, P and         C.

In the Co-base alloys, the total amount of Si, C and P will preferably not exceed 5% by weight. Typical quantitative fractions of the further alloy components that can be present in Co-base alloys in an amount significant for the alloy are stated in the following table 2:

TABLE 2 Further alloy components of Co-base alloys Alloy component Amount [% by weight] Cu   5 to 35 Fe 0.5 to 18 Ni   5 to 40 Mn 0.5 to 6 Cr   5 to 40 Mo   1 to 35 W 0.5 to 5 V   1 to 22 Nb   1 to 20 Ti 0.5 to 8 Si 0.1 to 3 C 0.1 to 4 P 0.1 to 3

Among the Co-base alloys of the second embodiment, preference is given in particular, to those which comprise 5 to 40% by weight in particular 7 to 30% by weight, of Cr as further alloy component. These alloys are termed hereinafter group 2.1. In addition to Cr, the base alloys of group 2.1 can comprise one or more of the following alloy components in an amount of up to 40% by weight, in particular up to 35% by weight: Fe, Ni, Mn, Cu, Mo, W, V, Nb, Ti, Si, C and P. Preferably, the further alloy component, where present, is present in an amount stated in table 2. Among the Co-base alloys of group 2.1, preference is given in particular to those that comprise Mo, W and/or Fe as further alloy component, in particular in an amount of in total 1 to 30% by weight.

Examples of Co-base alloys of group 2.1 are alloys of the compositions:

-   -   i. 53% by weight of Co, 31% by weight of Cr, 14% by weight of         Fe, 1.2% by weight of C (Stellite® 4),     -   ii. 65% by weight of Co, 28% by weight of Cr, 4.5% by weight of         W, 1.2% by weight of C, 1.1% by weight of Si (Stellite® 6),     -   iii. 66.5% by weight of Co, 28% by weight of Cr, 5% by weight of         Mo, 0.5% by weight of C (Stellite® 21),     -   iv. 58 to 62% by weight of Co, 25 to 30% by weight of Cr, 5 to         10% by weight of Mo, (Vitallium types, e.g. Haynes alloy 21),     -   v. 59% by weight of Co, 8.5% by weight of Cr, 29.5% by weight of         Mo, 2.1% by weight of Si (T 400).

A third embodiment of the invention relates to a process in which the anode material is an Fe-base alloy. Typical iron-base alloys are high-alloy stainless steels. They generally comprise substantially, i.e. at least 95% by weight, and in particular at least 98% by weight, and especially at least 99% by weight of:

-   -   a3) 50 to 95% by weight, in particular 55 to 95% by weight,         particularly preferably 55 to 90% by weight, and especially 58         to 90% by weight, of Fe and     -   b3) 5 to 50% by weight, in particular 5 to 45% by weight,         particularly preferably 10 to 45% by weight, and especially 10         to 42% by weight, of at least one further alloy component,         selected from Cu, Co, Ni, Mn, Cr, Mo, W, V, Nb, Ti, Si, P, S and         C.

In the Fe-base alloys, the total amount of Si, C and P will preferably not exceed 10% by weight. Typical quantitative fractions of the further alloy components that can be present in Fe-base alloys in an amount significant for the alloy are stated in table 3 hereinafter:

TABLE 3 Further alloy components of Fe-base alloys Alloy component Amount [% by weight] Cu   1 to 18 Co   1 to 23 Ni   5 to 45 Mn 0.2 to 2 Cr   3 to 30 Mo   1 to 30 V   1 to 22 Nb   1 to 20 Ti 0.1 to 1 Si 0.1 to 3 C 0.1 to 4 P 0.1 to 4 S 0.1 to 4

Among the Fe-base alloys of the third embodiment, preference is given, in particular, to chromium-comprising stainless steels which, in addition to the base metal, comprise Cr as alloy component, wherein the chromium content is generally in the range from 5 to 30% by weight, in particular 10 to 25% by weight. These alloys are termed hereinafter group 3.1. In addition to Cr, the base alloys of group 3.1 can comprise one or more of the following alloy components in an amount of up to 40% by weight, in particular up to 35% by weight: Co, Ni, Mn, Cu, Mo, V, Nb, Ti, Si, C, S and P. Preferably, the further alloy component, where present, is present in an amount stated in table 3. Among the Fe-base alloys of group 3.1, preference is given, in particular, to those that comprise Ni, Mo, V, Ti, Si and/or Nb as further alloy component, in particular in an amount of in total 1 to 30% by weight.

Examples of Fe-base alloys of group 3.1 are chromium steels, e.g. X12Cr13, X6Cr17 and X20Cr13, chromium-nickel steels, e.g. X2CrNi12, X5CrNi18-10, X8CrNiS18-9,

X2CrNi19-11, X2CrNi18-9, X10CrNi18-8, X1CrNi19-9, X2CrNiMo17-12-2, X2CrNiMo19-12, X2CrNiMo18-14-3, X2CrNiMoN18-14-3, X13CrNiMoN22-5-3, X6CrNiTi18-10, X6CrNiMoTi17-12-2, GX5CrNiMoNb19-11-2 and X15CrNiSi25-21 chromium-molybdenum steels, e.g. X12CrMoS17 and 25CrMo4 and also chromium-vanadium steels.

A fourth embodiment of the invention relates to a process in which the anode material is a Cu-base alloy. Typical copper-base alloys generally comprise substantially, i.e. at least 95% by weight, and in particular, at least 98% by weight, and especially at least 99% by weight of,

-   -   a4) 50 to 95% by weight, in particular 55 to 95% by weight,         particularly preferably 55 to 90% by weight, and especially 58         to 90% by weight, of Cu and     -   b4) 5 to 50% by weight, in particular 5 to 45% by weight,         particularly preferably 10 to 45% by weight, and especially 10         to 42% by weight, of at least one further alloy component,         selected among Ag, Pb, Ni and Zn.

Examples of Cu-base alloys of group 3.1 are nickel silver (alloy of 62% by weight of Cu, 18% by weight of Ni and 20% by weight of Zn) and Cupronickel (alloy of 75% by weight of Cu and 25% by weight of Ni).

In principle, as anode, any electrode type known to those skilled in the art can be used. These can consist completely of the respective base alloy or be a support electrode which has a support that is coated with the base alloy. Preference is given to electrodes which consist of the respective base alloy. The electrodes used as anode can be, for example, electrodes in the form of expanded metals, grids or metal plates.

As cathode, in principle, any electrode known to those skilled in the art as suitable for electrolysis of aqueous systems can be used. Since reduction processes take place at the cathode and the lignin is oxidized at the anode, when a heavy metal electrode is used such as, for example, a nickel cathode, the loading of the vanillin with this heavy metal is so low that the resultant vanillin can be used without problems in the food industry. Preferably, the electrode materials exhibit a low hydrogen overvoltage. Preference is given here to electrodes that have an electrode material selected among nickel, Ni-base alloys, Co-base alloys, Fe-base alloys, Cu-base alloys, silver, Ag-base alloys, i.e. silver-rich alloys having a silver content of at least 50% by weight, RuO_(x)TiO_(x)-mixed oxides, platinized titanium, platinum, graphite or carbon. In particular, the electrode material of the cathode is selected among Ni-base alloys, Co-base alloys, Fe-base alloys, Cu-base alloys, particularly preferably among Ni-base alloys, Co-base alloys and Fe-base alloys, and especially among the base alloys of groups 1.1, 1.2, 1.3, 2.1 and 3.1.

In principle, as cathode, any electrode type known to those skilled in the art can be used. This can comprise completely the respective electrode material or be a supported electrode that has an electrically conductive support which is coated with the electrode material. Preference is given to electrodes that comprise the respective electrode material, in particular one of the abovementioned base alloys, especially one of the base alloys of groups 1.1, 1.2, 1.3, 2.1 and 3.1. The electrodes used as cathode can be, for example, electrodes in the form of expanded metals, grids or metal plates.

The arrangement of anode and cathode is not restricted and comprises, for example, arrangements of planar gratings and/or plates which also can be arranged in the form of a plurality of stacks of alternating polarity and cylindrical arrangements of cylindrically formed grids, gratings or tubes which can also be arranged in the form of a plurality of cylinders alternating in polarity.

For achieving optimum space-time yields, various electrode geometries are known to those skilled in the art. Advantageous electrode geometries are a bipolar arrangement of a plurality of electrodes, an arrangement in which a rod-type anode is enclosed by a cylindrical cathode, or an arrangement in which both the cathode and the anode comprise a wire grid and these wire grids have been placed one over the other and rolled up cylindrically.

In one embodiment of the invention, the anode and cathode can be separated from one another by a separator. In principle, as separators, all separators usually used in electrolysis cells are suitable. The separator is typically a porous flat structure arranged between the electrodes, e.g. a grating, grid, woven fabric or nonwoven, made of an electrically non-conducting material which is inert under the electrolysis conditions, e.g. a plastic material, in particular a Teflon material, or a Teflon-coated plastic material.

For the electrolysis, any electrolysis cells known to those skilled in the art can be used, such as divided or undivided continuous-flow cells, capillary gap cells or plate stack cells. Particular preference is given to the undivided continuous-flow cell, e.g. a continuous-flow cell with circulation, in which the electrolyte is continuously conducted at the electrodes in circulation. The process can be carried out with good success both discontinuously and continuously.

The process according to the invention can likewise be carried out on an industrial scale. Corresponding electrolysis cells are known to those skilled in the art. All embodiments of this invention relate not only to the laboratory scale but also to the industrial scale.

In a preferred embodiment of the invention, the contents of the electrolysis cell are mixed. For this mixture of the cell contents, any mechanical agitator known to those skilled in the art can be used. The use of other mixing methods such as the use of Ultraturrax, ultrasound, jet nozzles or circulation or combinations of these measures is likewise preferred.

By applying the electrolysis voltage to the anodes and the cathodes an electric current is conducted through the electrolytes. In order to avoid side reactions such as overoxidation and detonating gas formation, generally a current density of 1000 mA/cm², in particular 100 mA/cm², will not be exceeded. The current densities at which the process is carried out are generally 1 to 1000 mA/cm², preferably 1 to 100 mA/cm². Particularly preferably, the process according to the invention is carried out at current densities between 1 and 50 mA/cm².

The total electrolysis time clearly depends on the electrolysis cell, the electrodes used and the current density. An optimum time can be determined by those skilled in the art by routine experiments, e.g. by sampling during the electrolysis.

In order to avoid a deposit on the electrodes, the polarity can be changed in short time intervals. The change in polarity can proceed at an interval of 30 seconds to 10 minutes. Preference is given to an interval of 30 seconds to 2 minutes. For this purpose, it is expedient that anode and cathode comprise the same material.

Processes known from the prior art must frequently be carried out at high pressures and at temperatures far above 100° C. This makes particular demands on the electrolysis cell, since this must be designed for superatmospheric pressure. Furthermore, both the electrolysis cell and the electrodes suffer under the corrosive conditions which are established at a high temperature. In the process according to the invention, it is not necessary to work at high pressures and temperatures.

The electrolysis is carried out according to the process according to the invention generally at a temperature in a range from 0 to 100° C., preferably 50 to 95° C., in particular 70 to 90° C.

In the process according to the invention, the electrolysis is generally carried out at a pressure below 2000 kPa, preferably below 1000 kPa, in particular below 150 kPa, e.g. in the range from 50 to 1000 kPa, in particular 80 to 150 kPa. Particularly preferably, the process according to the invention is to be carried out at a pressure in the range of atmospheric pressure (101±20 kPa).

In a particularly preferred embodiment, the process according to the invention is carried out at a temperature in the range from 50 to 95° C., in particular 70 to 90° C., and in the range of atmospheric pressure (101±20 kPa).

The aqueous, lignin-comprising suspension or solution generally comprises 0.5 to 30% by weight, preferably 1 to 15% by weight, in particular 1 to 10% by weight, of lignin, based on the total weight of the aqueous, lignin-comprising suspension or solution.

According to the invention, for the preparation of vanillin, an aqueous, alkaline suspension or solution is electrolyzed. Aqueous, alkaline lignin-comprising solutions or suspensions are here, and hereinafter, taken to mean an aqueous solution or suspension which comprises lignin or lignin derivatives, for example lignin sulfate, lignin sulfonate, kraft lignin, alkali lignin or organosolv lignin or mixtures thereof, as lignin component and which has an alkaline pH, preferably a pH of at least 10, in particular at least 12, and especially at least 13.

The aqueous, alkaline solution or suspension can be an aqueous solution or suspension which arises as a byproduct in an industrial process, such as in the preparation of paperstock, pulp or cellulose, e.g. black liquor, and also the lignin-comprising waste water streams from the sulfite process, from the sulfate process, from the organocell or organosolv process, from the ASAM process, from the kraft process or from the natural pulping process. The aqueous, alkaline solution or suspension can be an aqueous solution or suspension that is prepared by dissolving a lignin or lignin derivative in an aqueous alkali or in water with addition of a base, e.g. lignin sulfate, lignin sulfonate, kraft lignin, alkali lignin or organosolv lignin, or a lignin which arises in an industrial process such as the preparation of paperstock, pulp or cellulose, e.g. lignin from black liquor, from the sulfite process, from the sulfate process, from the organocell or organosolv process, from the ASAM process, from the kraft process or from the natural pulping process.

In all processes of the preparation of paper, pulp or cellulose, lignin-comprising wastewater streams arise. These can be used as aqueous, lignin-comprising suspension or solution in the process according to the invention, optionally after setting an alkaline pH. The wastewater streams of the sulfite process for paper preparation frequently comprise lignin as lignin sulfonic acid. Lignin sulfonic acid can be used directly in the process according to the invention or after alkaline hydrolysis. In the sulfate process or kraft process, lignin-comprising wastewater streams arise, e.g. in the form of black liquor. In the organocell process which, owing to its environmental friendliness, will achieve greater importance in the future, the lignin arises as organosolv lignin. Lignin sulfonic acid-comprising or organosolv lignin-comprising wastewater streams and also black liquor are particularly suitable as aqueous, alkaline lignin-comprising suspensions or solutions for the process according to the invention.

Alternatively, the aqueous, lignin-comprising suspensions or solutions may also be prepared by dissolving or suspending at least one lignin-comprising material in aqueous alkali, i.e. in an aqueous solution of a suitable base or in water with addition of base. The lignin-comprising material preferably comprises at least 10% by weight, in particular at least 15% by weight, and in particular preferably at least 20% by weight, of lignin, based on the total weight of the lignin-comprising material. The lignin-comprising material is preferably selected among kraft lignin, lignin sulfonate, oxidized lignin, organosolv lignin or other lignin-comprising residues from the paper industry or fiber production, in particular among kraft lignin, lignin sulfonate and oxidized lignin which arises in an electrochemical oxidation of non-oxidized lignin.

As bases for setting the pH of the aqueous, alkaline lignin-comprising suspension or solution, especially inorganic bases can be used, e.g. alkali metal hydroxides such as NaOH or KOH, ammonium salts such as ammonium hydroxide and alkali metal carbonates such as sodium carbonate, e.g. in the form of soda. Preference is given to alkali metal hydroxides, in particular NaOH and KOH. The concentration of inorganic bases in the aqueous, lignin-comprising suspension or solution should not exceed 5 mol/l, and in particular 4 mol/l, and is typically in the range from 0.01 to 5 mol/l, and in particular in the range from 0.1 to 4 mol/l.

In a preferred embodiment, oxidized lignin is used that originates from a previous electrolysis cycle. It has proved to be advantageous in this case to use oxidized lignin in at least one further electrolysis cycle, preferably in at least two further electrolysis cycles, and in particular in at least three further electrolysis cycles. This repeated use of the oxidized lignin is advantageous in that vanillin can be isolated repeatedly. Therefore, the yield of vanillin, based on the amount of lignin originally used, is markedly increased and therefore the economic efficiency of the overall process is increased. Furthermore, owing to the repeated use of the oxidized lignin, the concentration of the oxidation-sensitive vanillin can be kept so low in the electrolyte per oxidation operation, that the unwanted side reactions such as overoxidation can be effectively repressed, whereas the overall yield of vanillin increases over the overall process (a plurality of electrolysis cycles).

Accordingly, further preferred embodiments relate to a process according to the invention in which the aqueous, alkaline lignin-comprising suspension or solution is selected among

-   -   a) an aqueous suspension or solution which is prepared by         depleting vanillin from an aqueous reaction mixture, wherein the         reaction mixture was obtained by oxidation, in particular by         electrolysis, of an aqueous, alkaline lignin-comprising         suspension or solution, and     -   b) an aqueous suspension or solution which is prepared by         dissolving or suspending oxidized lignin in aqueous alkali,         wherein oxidized lignin was obtained by oxidation, in particular         by electrolysis, of an aqueous, alkaline lignin-comprising         suspension or solution.

It is particularly preferred to use wastewater streams or residues from the preparation of paper and pulp, in particular black liquor or kraft lignin. Accordingly, further preferred embodiments relate to a process according to the invention in which the aqueous, alkaline lignin-comprising suspension or solution is selected among wastewater streams from the preparation of paper and pulp, in particular black liquor, or solutions of kraft lignin.

At high lignin concentrations in the aqueous, lignin-comprising suspension or solution, the viscosity of the solution or suspension can increase greatly and the solubility of the lignin can become very low. In these cases, it can be advantageous, before the electrochemical oxidation, to carry out a prehydrolysis of the lignin, whereby the solubility of the lignin is improved and the viscosity of the aqueous, lignin-comprising suspension or solution is decreased. Typically, for the prehydrolysis of lignin, it is heated in an aqueous alkali metal hydroxide solution to above 100° C. The concentration of the alkali metal hydroxide is preferably 0.5 to 5 mol/l, in particular 1.0 to 3.5 mol/l. Preferably sodium hydroxide or potassium hydroxide is used. In a preferred embodiment of the prehydrolysis process, the lignin-comprising alkali metal hydroxide solution is heated to a temperature of 150 to 250° C., in particular 170 to 190° C., and stirred vigorously for 1 to 10 h, preferably 2 to 4 h. The prehydrolyzed lignin can be separated off from the alkali metal hydroxide solution before the electrochemical oxidation. Alternatively, there is the possibility of carrying out the electrochemical oxidation directly with the lignin-comprising alkali metal hydroxide solution.

The aqueous, alkaline lignin-comprising suspension or solution can comprise a conducting salt for improving the conductivity. This generally concerns alkali metal salts such as salts of Li, Na, K or quaternary ammonium salts such as tetra(C₁-C6-alkyl)ammonium or tri(C₁-C₆-alkyl)methylammonium salts. Counterions which come into consideration are sulfate, hydrogensulfate, alkylsulfates, arylsulfates, halides, phosphates, carbonates, alkylphosphates, alkylcarbonates, nitrate, alkoholates, tetrafluoroborate, hexafluorophosphate, perchlorate, bistriflates and bistriflimide.

In addition, as conducting salts, ionic liquids are also suitable. Suitable electrochemically stable ionic liquids are described in “Ionic Liquids in Synthesis”, editors Peter Wasserscheid, Tom Welton, Wiley-VCH 2003, chapters 1 to 3.

For the electrochemical oxidation of the lignin, a metal-comprising or metal-free mediator can be added to the aqueous, alkaline lignin-comprising suspension or solution. Mediators are taken to mean redox pairs which allow an indirect electrochemical oxidation. The mediator is converted electrochemically to the higher oxidation state, then acts as oxidizing agent and is regenerated thereafter by electrochemical oxidation. This is therefore an indirect electrochemical oxidation of the organic compound, since the mediator is the oxidizing agent. The oxidation of the organic compound with the mediator in the oxidized form can be carried out in this case in the electrolysis cell in which the mediator was converted to the oxidized form, or in one or more separate reactors (“ex-cell process”). The latter method has the advantage that any remaining traces of the organic compound that is to be oxidized do not interfere in the preparation or regeneration of the mediator.

Suitable mediators are compounds which can exist in two oxidation states, act in the higher oxidation state as an oxidizing agent, and can be regenerated electrochemically. As mediators, e.g. salts or complexes of the following redox pairs can be used: Ce (III/IV), Cr (II/III), Cr (III/VI), Ti (II/III), V (II/III), V (III/IV), V (IV/V), Ag (I/II), AgO⁺/AgO⁻, Cu (I/II), Sn (II/IV), Co (II/III), Mn (II/III), Mn (II/IV), Os (IV/VIII), Os (III/IV), Br₂/Br⁻/BrO₃, I-/I₂, I₃ ⁺/I₂ IO₃ ⁺/IO₄ ⁻, Fremy's salt (dipotassium nitrosodisulfonate) or else organic mediators, such as ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), TEMPO, viologens such as violuric acid, NAD⁺/NADH, NADP⁺/NADPH, wherein the stated systems can also be metal complexes with diverse ligands or else solvent ligands, such as, e.g., H2O, NH₃, CN⁻, OH⁻, SCN⁻, halogens, O₂, acetylacetonate, dipyridyl, phenanthroline or 1,10-phenanthroline 5,6-dione. Preferably, in the process according to the invention, transition metal-free mediators, e.g. nitrosodisulfonates such as Fremy's salt (dipotassium nitrosodisulfonate), are used. The mediator is preferably used in amounts of 0.1 to 30% by weight, particularly preferably from 1 to 20% by weight, based on the total weight of the aqueous, lignin-comprising suspension or solution.

In a particularly preferred embodiment, the process according to the invention is carried out without addition of mediators.

The aqueous, alkaline lignin-comprising suspension or solution can additionally comprise an inert solvent. Suitable solvents are polar-aprotic solvents having a high electrochemical stability such as acetonitrile, propionitrile, adiponitrile, suberonitrile, propylene carbonate, ethylene carbonate, N-methylpyrrolidone, hexamethylphosphoramide, dimethyl sulfoxide and dimethylpropyleneurea (DMPU). Further suitable polar-aprotic solvents are described in Kosuke Izutsu, “Electrochemistry in Nonaqueous Solutions”, Wiley-VCH 2002, chapter 1.

In the process according to the invention, generally inert solvents are used in an amount not greater than 60% by weight, preferably not greater than 30% by weight, in particular not greater than 20% by weight, e.g. 2.5 to 30% by weight, or 5 to 20% by weight, based on the total amount of the aqueous, lignin-comprising suspension or solution used.

The vanillin obtained by the process according to the invention can be isolated from the aqueous, lignin-comprising solution by methods known to those skilled in the art. For example, the vanillin formed in the electrolysis can be withdrawn or depleted from the aqueous, lignin-comprising suspension or solution by distillation or extraction.

Distillation methods that are suitable are distillation processes known to those skilled in the art such as, e.g. vacuum distillation, distillation under a protective gas atmosphere, or steam distillation. An advantage of vanillin separation via distillation processes is that the vanillin is not brought into contact with organic solvents that are potentially hazardous to health.

Vanillin can likewise be removed from the aqueous, lignin-comprising suspension or solution by extraction. This is particularly advantageous, since the sensitive vanillin is not exposed to a further thermal stress. Extraction processes that are known to those skilled in the art are suitable therefor.

The aqueous, lignin-comprising suspension or solution can be admixed, e.g. with an organic solvent, for the extraction, in order in this manner to separate off the vanillin formed (liquid-liquid extraction). Suitable organic solvents are water-immiscible organic solvents, e.g. hydrocarbons having 5 to 12 carbon atoms, such as hexane or octane, chlorinated hydrocarbons having 1 to 10 carbon atoms, such as dichloromethane or chloroform, aliphatic ethers having 2 to 10 carbon atoms, such as diethyl ether or diisopropyl ether, cyclic ether, or aliphatic esters such as ethanoic acid ethyl ester. Preference is given to halogen-free organic solvents. In addition, it is possible to extract vanillin using supercritical fluids. In particular, supercritical CO₂ is suitable therefor.

The lignin formed can likewise be removed from the aqueous, lignin-comprising suspension or solution by solid-phase extraction. For this purpose, solid-phase extraction media are added to the aqueous, lignin-comprising suspension or solution. The vanillin (vanillate) adsorbed to the extraction medium can then be eluted from the solid phase using polar organic solvents such as methanol, for example, that are known to those skilled in the art. Furthermore, solid-phase extraction is also possible in a similar manner to solid-phase synthesis. In this case, the vanillin is covalently bound to the solid phase as vanillate. After separating off the solid phases from the aqueous, lignin-comprising suspension or solution, the vanillin is liberated again by breaking the covalent bond. In both cases, a concentrated crude product is obtained which can then be purified and isolated by distillation in a simple manner.

In a preferred embodiment of the process according to the invention, the vanillin that is generated is isolated from the aqueous, alkaline lignin-comprising solution or suspension (hereinafter alkaline electrolysate) obtained in the electrolysis by treatment with a basic adsorbent, in particular an anion exchanger. Since in the alkaline electrolysate the vanillin is present in anionic form as vanillate, it is adsorbed by the basic adsorbent, for example an anion exchanger, and can then be liberated by treating the vanillate-loaded anion exchanger with acid, preferably a dilute solution of a mineral acid or an organic acid in an organic solvent, or in an aqueous-organic solvent mixture.

For example, the adsorbent, e.g. the anion exchanger, can be added to the alkaline electrolysate obtained in the electrolysis, after a certain residence time the adsorbent, e.g. the anion exchanger, can be separated off from the alkaline electrolysate and then the vanillin adsorbed by the adsorbent can be liberated by treatment of the adsorbent with acid. Preferably, the alkaline electrolysate is first passed through a bed of the adsorbent, in particular a bed of an anionic exchanger, for example through one or more columns packed with the adsorbent, e.g. an anion exchanger, and then pass a dilute solution of an acid in particular a mineral acid or an organic acid, through the bed of the adsorbent, and elute the vanillin in the course of this.

Suitable absorbents are in principle all substances which have basic groups or are treated with hydroxide ions. These include alkalized activated carbons, basic aluminum oxides, clays, basic adsorber resins, in particular anionic exchangers or anion-exchange resins. Anion exchangers or anion exchanger resins generally have functional groups which are selected from tertiary amino groups, quaternary ammonium groups and quaternary phosphonium groups.

The anion exchangers preferably used for this purpose are generally crosslinked organic polymer resins which have quaternary ammonium groups or phosphonium groups. Preferably, the anion exchangers preferably used are those from the group of crosslinked polystyrene resins, where a part of the phenyl rings of the crosslinked polystyrene bear quaternary ammonium groups, for example, trialkylammonium groups bound via alkylene groups, especially trimethylammonium groups bound via a methylene group. Organic polymer resins suitable for this purpose as anion exchangers are also crosslinked polyvinylpyridines, in which some of the pyridine groups are quaternized, for example as 1-alkylpyridinium group, especially as 1-methylpyridinium groups, and also crosslinked acrylate resins which bear trialkylammonium groups, bound via alkylene groups, especially trimethylammonium groups bound via a 1,2-ethanediyl or 1,3-propanediyl. Typically, the charge density, i.e. the number of ionic groups in anion exchanger suitable according to the invention, is in the range from 0.5 to 6 mmol/g, in particular 1 to 5 mmol/g of ion-exchange resin or 0.1 to 3 eq/l (mole equivalents per liter, moist). Suitable adsorbents are also polymers which N-C₁-C₈-alkylimidazolium groups. In these polymers, the N-C₁-C₈-alkylimidazolium group are bound directly or via a spacer to the polymer backbone. Such polymers can be obtained by polymer-analogous reaction with N-C₁-C₈-alkylimidazole compounds, for example by reacting polymers having haloalkyl groups, in particular chlorobenzyl groups, e.g. copolymers of styrene and chloromethylstyrene, with N-C₁-C₈-alkylimidazoles. It is likewise possible to produce such polymers by homo- or copolymerization of monomers having imidazolium groups, for example (N-C₁-C₈-alkylimidazolium)methylstyrene, N-vinyl-N-C₁-C₈-alkylimidazolium, ω-(N-C₁-C₈-alkylimidazolium)-C₂-C₈-alkyl acrylate or ω-(N-C₁-C₈-alkylimidazolium)-C₂-C₈-alkyl methacrylate, optionally with comonomers such as C₁-C₈-alkyl acrylates, C₁-C₈-alkyl methacrylates, C₂-C₈-hydroxyalkyl acrylates, C₂-C₈-hydroxyalkyl methacrylates or styrene, for example by free-radical polymerization or by controlled radical polymerization such as RAFT or ATRP. Such polymers are known and are described, for example, by J. Yuan, M. Antonietti, Polymer 2011, 52, 1469-1482; J. Huang, C. Tao, Q. An, W. Zhang, Y. Wu, X. Li, D. Shen, G. Li, Chem. Comm. 2010, 46, 967; R. Marcilla, J. Alberto Blazquez, J. Rodriguez, J. A. Pomposo, D. Mecerreyes, J. Pol. Sci. A: Pol. Chem. 2004, 42, 208-212; J. Tang, H. Tang, W. Sun, M. Radosz, Y. Shen, J. Pol. Sci. A: Pol. Chem. 2005, 43, 5477-5489; J. Tang, Y. Shen, M. Radosz, W. Sun, Ind. Eng. Chem. Res. 2009, 48, 9113-9118.

For elution of the vanillin from the basic adsorbent (for example an anion exchanger), especially dilute solutions of mineral acids such as hydrochloric acid, sulfuric acid or phosphoric acid, in organic solvents are suitable, and also dilute solutions of mineral acids in organic-aqueous solvent mixtures. For elution of the vanillin from the basic adsorbent (e.g. an anion exchanger), especially dilute solutions of organic acids such as trifluoromethanesulfonic acid, acetic acid, formic acid or propionic acid in organic solvents are suitable, and also dilute solutions of organic acids in organic-aqueous solvent mixtures.

Suitable organic solvents are especially those which are miscible unrestrictedly with water at 22° C. or at least, at 22° C., dissolve in water in an amount of at least 200 g/I. These include, especially, dimethyl sulfoxide, acetone, C₁-C₄ alkanols such as methanol, ethanol, isopropanol, n-propanol, 1-butanol, 2-butanol and tert-butanol, alkanediols, such as glycol and 1,4-butanediol, glycerol, but also cyclic ethers such as dioxane, methyltetrahydrofuran or tetrahydrofuran, nitrogen heterocycles, such as pyridine or N-methylpyrrolidine and mixtures. Preference is given to C₁-C₄ alkanols, and especially methanol.

Suitable acids are especially mineral acids such as hydrochloric acid, phosphoric acid and in particular sulfuric acid and organic acids such as methanesulfonic acid, formic acid, acetic acid and propionic acid. Preferably, the solution of the acid has a concentration of acid in the range from 0.01 to 10 mol kg⁻¹, in particular 0.1 to 5 mol kg⁻¹.

The eluate arising during the elution can be subjected to further purification steps, for example a crystallization, filtration or chromatography.

In addition, it is possible to decrease the fraction of volatile components of the electrolysate before separating off the vanillin by distillation. Then, the vanillin can be extracted from the remaining residue using the abovementioned extraction media.

The vanillin can be separated off continuously or discontinuously. It is particularly advantageous to remove the vanillin from the aqueous, lignin-comprising suspension or solution during the electrochemical oxidation continuously or at intervals. For this purpose, for example, a substream of the electrolysate can be discharged from the electrolysis arrangement and the lignin present therein can be depleted, for example by continuous (solid-phase) extraction or by steam distillation. Also, the electrolysis can be interrupted once or repeatedly, during the interruption the electrolysate can be subjected to depletion of the vanillin, as described above, and then the electrolysis can be continued. In a special embodiment, the vanillin is depleted continuously or at intervals from the electrolysate using an anion exchanger. This succeeds, for example, by discharging a substream from the electrolysis arrangement during the electrolysis and treating it with the ion exchanger, e.g. by passing it through a bed of the anion exchanger. Also, the electrolysate can be treated with the anion exchanger during an interruption in electrolysis and the electrolysis can be continued after the interruption. In this manner, the vanillin is depleted from the electrolysate at intervals.

Since the anode materials used in the process according to the invention do not exhibit any significant corrosion under the reaction conditions, the vanillin that is prepared in this manner has no, or no significant, heavy metal pollution and can therefore be used in the food industry. The invention therefore further relates to the use of the vanillin which was obtained by the process according to the invention as aroma substance in the food industry.

After termination of the electrolysis, the aqueous, lignin-comprising suspension or solution, in addition to the vanillin formed, still comprises oxidized lignin. After separating off the vanillin and optionally other low-molecular-weight products, the oxidized lignin can be obtained by drying the aqueous, lignin-comprising solution. A lignin prepared in this manner can be used, for example, advantageously as additive in the construction industry, for example as cement or concrete additive.

The examples hereinafter are intended to describe the invention further and are not to be understood as restrictive.

Analysis

For the gas-chromatographic analysis of the electrolysis products, the stationary phase used was an HP-5 column from Agilent of 30 m length, 0.25 mm diameter and 1 μm coating thickness. This column is heated by a temperature program from 50° C. in the course of 10 min at 10° C./min to 290° C. This temperature is maintained for 15 min. The carrier gas used was hydrogen at a flow rate of 46.5 ml/min.

Electrode Materials:

Electrode material Composition Monel 400 K 65% by weight Ni, 30% Cu, 2% by weight Fe Monel 500 K 63% by weight Ni, 30% Cu, 2% by weight Fe, 1.5% by weight Mn, 0.5% by weight Ti Hastelloy ® C 276 57% by weight Ni, 17% by weight Mo, 16% by weight Cr and Fe, W and Mn Inconel ® 625 61% by weight Ni, 9% by weight Mo, 22% by weight Cr, 5% by weight Fe Nickel silver 62% by weight Cu, 18% by weight Ni, 20% by weight Zn Cupronickel 75% by weight Cu, 25% by weight Ni NiCrFe 72-76% by weight Ni, 18-21% by weight Cr, 0.08-0.13% by weight C, 5% by weight Fe Stellite ® 4 53% by weight Co, 31% by weight Cr, 14% by weight Fe, 1.2% by weight C Stellite ® 6 65% by weight Co, 28% by weight Cr, 4.5% by weight W, 1.2% by weight C, 1.1% by weight Si Stellite ® 21 66.5% by weight Co, 28% by weight Cr, 5% by weight Mo, 0.5% by weight C T400 59% by weight Co, 8.5% by weight Cr, 29.5% by weight Mo, 2.1% by weight Si

Electrolysis

Examples 1 to 4 Electrolysis of a Lignin Solution at Ni—Cu Electrodes

525 to 526 mg of kraft lignin were dissolved in 85 g of the respective electrolyte in a temperature-controllable, undivided cell with stirring. The cell had two electrodes that were arranged in the cell at a distance of 0.5 cm. The two electrodes were plates (thickness: 3 mm) in each case made of a copper-comprising Ni-base alloy (Monel 400), having dimensions 3.0×3.3 cm². The solution was electrolyzed at a current density of 1.9 mA/cm² and a temperature of 80° C. for 20.6 hours (Q=1411 C). The maximum terminal voltage during the reaction was 3.3 V. After the amount of charge flowed through, the cell contents were cooled to room temperature, admixed with a known amount of a standard (n-hexadecane) and filtered from any solids present. Then, the solution was adjusted to pH=1 to 2 using concentrated hydrochloric acid and admixed with 20 ml of dichloromethane. The gelatinous solid that had precipitated out was filtered over kieselguhr and washed with approximately 25 ml of dichloromethane. The organic phase was separated off. The aqueous phase was extracted three times more each time with 80 ml of dichloromethane. The combined organic phases were washed with 50 ml of saturated common salt solution and then dried over Na₂SO₄. After the solvent was removed under reduced pressure, an oily, mostly goldish-brown residue remained, which was analyzed with respect to its composition by gas chromatography. The gas-chromatographic analysis of the organic crude products gave typical compositions, based on lignin used (% by weight) that are summarized in table 4.

TABLE 4 Vanillin [% by Acetovanillone [% by Example Electrolyte weight]¹⁾ weight]¹⁾ 1 3M aqueous NaOH 2.15 0.25 2 2M aqueous NaOH 1.64 0.15 3 1M aqueous NaOH 1.52 0.09 4 3M aqueous KOH 1.55 0.14 ¹⁾The yield was determined by gas chromatography by adding n-hexadecane as internal standard, based on kraft lignin used.

Examples 5 to 10 Electrolysis of a Lignin Solution at Electrodes made of Ni— or Cu-Base Alloys

The electrolysis was carried out in a similar manner to example 1 with the following change: the electrolyte used was 3 M aqueous sodium hydroxide solution. As electrodes, plates were used (thickness: 3 mm) made of various Ni— and Cu-base alloys (see table 5) having dimensions of 3.0×4.0 cm², that were arranged at a distance of 0.5 cm to one another. The solution was electrolyzed for 17.2 hours (Q=1411 C). The maximum cell voltage during the electrolysis was 2.9 V. The results are summarized in table 5.

TABLE 5 Vanillin [% by Acetovanillone [% by Example Electrode material weight]¹⁾ weight]¹⁾ 5 Monel 500 K 0.57 0.1 6 Hastelloy C 276 1.45 0.15 7 Inconel 625 0.91 0.07 8 Nickel silver 1.23 0.21 9 Cupronickel 0.62 0.06 10 NiCrFe 0.76 0.08 ¹⁾The yield was determined by gas chromatography by adding n-hexadecane as internal standard, based on kraft lignin used.

Examples 11 to 14 Electrolysis of a Lignin Solution at Electrodes made of Co-Base Alloys

The electrolysis was carried out in a similar manner to example 1 with the following change:

As electrolyte, 3 M aqueous sodium hydroxide solution was used. The electrodes used were plates (thickness: 3 mm) made of various Co-base alloys (see table 6) (dimensions 3.0×4.0 cm²) having a maximally utilizable electrode surface area of 9 cm², that were arranged at a distance of 0.5 cm from one another. The solution was electrolyzed for 23 h (Q=1411 C). The maximum cell voltage during the electrolysis was 2.9 V. The results are summarized in table 6.

TABLE 6 Vanillin [% by Acetovanillone [% by Example Electrode material weight]¹⁾ weight]¹⁾ 11 Stellite ® 4 2.19 0.1 12 Stellite ® 6 1.90 0.23 13 Stellite ® 21 3.16 0.39 14 T400 2.59 0.44 ¹⁾The yield was determined by gas chromatography by adding n-hexadecane as internal standard, based on kraft lignin used.

Comparative Examples C1 and C2 Electrolysis of a Lignin Solution at Electrodes made of Co

The electrolysis was carried out in a similar manner to example 1 with the following change:

The electrolyte used was 3 M aqueous sodium hydroxide solution. The electrodes used were plates (thickness: 1 mm) made of Co (dimension 3.0×4.0 cm²) having a maximally utilizable electrode surface area of 9 cm², that were arranged at a distance of 0.5 cm from one another. The solution was electrolyzed for 17.2 hours (Q=1411 C). The maximum cell voltage during the electrolysis was 3.1 V.

As a result of the electrolysis of comparative example 1, a black layer formed on the anode. The anode was used in a second electrolysis under otherwise identical conditions (comparative example C2). The results are summarized in table 7.

TABLE 7 Vanillin [% by Acetovanillone Guaiacol [% Example Electrode material weight]¹⁾ [% by weight]¹⁾ by weight]¹⁾ C1 Cobalt > 99% 2.13 0.31 — C2²⁾ Cobalt > 99%²⁾ 0.68 0.15 0.33 ¹⁾The yield was determined by gas chromatograph by adding n-hexadecane as internal standard, based on kraft lignin used. ²⁾Electrode of comparative example 1 without removal of the oxide layer formed.

Example 15

2.011 g of kraft lignin were placed in a one-pot cell (V=600 ml) without cooling jacket and dissolved in 300 g of 3 M NaOH with stirring. 11 plates of Monel 400K (4.9 cm×2.1 cm) were connected in a bipolar manner at a spacing of 0.3 cm, in such a manner that the cell comprised ten half-chambers. The solution was electrolyzed for approximately 7.8 hours (Q=560 C; based on electrolyte: Q=5600 C). The cell voltage that was established was in the range from 3.0 to 3.1 V. After the amount of charge flowed through, the cell contents were brought to room temperature and applied over a column bed of Amberlite IRA402(OH) (m_(Amberlite=)40 g, d_(column)=2 cm, h=20 cm). The ion exchanger used had been swollen in advance for several hours in water. After the reaction solution had completely passed through the column material (droplet rate: 1 drop/sec), the filtrate was electrolyzed again under the above-described conditions. In total, the solution was electrolyzed and filtered five times.

For isolating the vanillin adsorbed by the ion exchanger, the anion exchanger was washed in portions using a 2% strength solution of HCl in MeOH (V_(tot)=350 ml, droplet rate: 1 drop/sec). The resultant filtrate was admixed with 100 ml of H₂O and extracted three times, each time with 150 ml of dichloromethane. The combined organic phases were washed with approximately 100 ml of saturated common salt solution, dried over Na₂SO₄ and freed from the solvent under reduced pressure. A bronze-colored foam remained, which was purified by column chromatography (d=2 cm, h=20 cm, of silica gel 60) (eluent:cyclohexane/ethyl acetate in a volumetric ratio of 3:2). Based on kraft lignin used, 2.47% by weight of vanillin were obtained, contaminated with 8% acetovanillone (GC fraction).

For workup of the filtrate, it was acidified with concentrated hydrochloric acid with cooling and the acidified filtrate was filtered through a bed of kieselguhr, in order to remove lignin that had precipitated out. The kieselguhr bed was thoroughly rinsed with dichloromethane. The aqueous phase was extracted three times, each time with 150 ml of dichloromethane. The combined organic phases were washed with 100 ml of saturated common salt solution, dried over Na2SO₄ and freed from the solvent under reduced pressure. A viscous solid (m_(RP)=11.9 mg, 0.59% by weight, based on kraft lignin used), remained. Gas-chromatographic analysis gave the following typical composition (GC fractions): 75.2% vanillin, 11.0% acetovanillone.

Comparative Example 3 Electrolysis of a Lignin Solution at Nickel Electrodes

525-530 mg of Kraft lignin were dissolved in 85 g of 3 M NaOH in a temperature-controllable undivided cell with stirring. Both anode and cathode were Ni meshes ((weave: upside twill weave 555, mesh: 124, mesh width: 0.125, wire diameter: 0.080, material: elemental nickel (2.4066-Ni), manufacturer: GKD, Article No: 29230125; 3.0×4.0 cm²). The electrodes were arranged in parallel to one another at a distance of approximately 0.3 cm and immersed in the electrolyte solution. The electrolyte solution was electrolyzed at various current densities and at a temperature of 80° C. In this case, a charge quantity of 1411 C was applied. The maximum terminal voltage during the reaction was 4.1 V. After the charge quantity had flowed through, the cell contents were cooled to room temperature. Then, the electrolyzed solution was adjusted to pH=1-2 with 50% strength H2SO₄ and admixed with 20 ml of dichloromethane. The precipitated, gelatinous solid was filtered over kieselguhr and rewashed with approximately 25 ml of dichloromethane. The organic phase was separated off. The aqueous phase was extracted a further three times each time with 80 ml of dichloromethane. The combined organic phases were washed with 50 ml of saturated sodium.chloride solution and then dried over Na₂SO₄. After the solvent was removed under reduced pressure, an oily, mostly gold-brown residue remained. This was dissolved in approximately 1 ml of ethyl acetate and admixed with 2 μl of the internal standard 1-phenyldodecane. The solution was filtered through cotton wool and examined with respect to its composition by gas chromatography. The analysis of the organic crude products gave typical compositions, based on lignin used (% by weight) which are summarized in Table 8.

TABLE 8 Current density Vanillin Acetovanillone Example [mA/cm²] [% by weight] [% by weight]¹⁾ V3a 1.9 0.8 0.1 V3b 3.8 0.7 0.1 V3c 9.5 0.9 0.1 V3d 19 0.7 0.1 V3e 38 0.8 0.1

Example 16 Electrolysis of a Lignin Solution at Stainless Steel Electrodes

The electrolysis was carried out in a similar manner to comparative example 1 with the following change. As electrodes, stainless steel meshes were used (weave: reversed twill weave 555, mesh: 200, Mw: 0.077, wire diameter: 0.050, material: 1.4404, manufacturer: GKD, Article No: 29370850; 3.0×4.0 cm²).

Analysis of the organic crude products gave typical compositions, based on lignin used (% by weight) which are summarized in Table 9.

TABLE 9 Current density Vanillin Acetovanillone Example [mA/cm²] [% by weight] [% by weight]¹⁾ 16a 1.9 0.9 0.1 16b 3.8 1.0 0.1 16c 9.5 1.0 0.1 16d 19 1.1 0.1 16e 38 1.1 0.1

Comparative Example 4

The procedure was carried out in a similar manner to example 1 with the following variation: 525-526 mg of Kraft lignin were dissolved in 85 g of electrolyte in an undivided call with stirring. As electrolyte, 3 M aqueous sodium hydroxide solution was used. The cell was provided with an anode and a cathode which consisted of platinum and had a maximum useable electrode surface area of approximately 12 cm². The electrodes were mounted in parallel at a distance of 0.5 cm and the solution was then electrolyzed for 18 h (Q=1411 C). The maximum cell voltage during the reaction was 3.1 V. As a consequence of the reaction, no surface change and also no loss in mass at the anode or cathode could be observed. The yield of vanillin was 0.48% by weight, based on Kraft lignin used, the yield of acetovanillone was 0.06% by weight.

Comparative Example 5

The procedure was carried out in a similar manner to example 1 with the following variation: 525-526 mg of Kraft lignin were dissolved in 85 g of electrolyte (3 M aqueous NaOH) in an undivided cell with stirring. The cell was provided with an anode and a cathode which consisted of platinum and had a maximum useable electrode surface area of approximately 12 cm². The electrodes were mounted in parallel at a distance of 0.5 cm and the solution was then electrolyzed for 18 h (Q=1411 C). The maximum cell voltage during the reaction was 2.7 V. As a consequence of the reaction, a change in the anode surface in the form of a mat-yellow layer was observed. This friable layer could easily be removed by treatment with a sparing amount of water and the original appearance of the copper reappeared. A loss in mass of 535 mg occurred at the anode because of corrosion. The yield of vanillin was 1.99% by weight, based on Kraft lignin used the yield of acetovanillone was 0.09% by weight. 

1. A process for preparing vanillin, said process comprising an electrolysis of an aqueous, alkaline lignin-comprising suspension or solution, wherein, as anode material, a base alloy is used which is selected from Co-base alloys, Fe-base alloys, Cu-base alloys, or Ni-base alloys.
 2. The process according to claim 1, wherein, as anode material, a base alloy is used which is selected from Co-base alloys, Fe-base alloys, or Ni-base alloys, wherein (1) the Ni-base alloy comprises: a1) 50 to 95% by weight of Ni and b1) 5 to 50% by weight of at least one further alloy component, selected from Cu, Fe, Co, Mn, Cr, Mo, V, Nb, Ti, Si, Al, C, or S; (2) the Co-base alloy substantially: a2) 50 to 95% by weight of Co and b2) 5 to 50% by weight of at least one further alloy component, selected from Cu, Fe, Ni, Mn, Cr, Mo, W, V, Nb, Ti, Si, P, or C; and (3) the Fe-base alloy substantially comprises: a3) 50 to 95% by weight of Co and b3) 5 to 50% by weight of at least one further alloy component, selected from Cu, Co, Ni, Mn, Cr, Mo, W, V, Nb, Ti, Si, P, S or C.
 3. The process according to claim 2, wherein the base alloy is selected from 1.1 Ni-base alloys that comprise 5 to 35% by weight of Cu, 1.2 Ni-base alloys that comprise 5 to 40% by weight of Cr, 1.3 Ni-base alloys that comprise 5 to 35% by weight of Mo, 2.1 Co-base alloys that comprise 5 to 40% by weight of Cr, or 3.1 high-alloy chromium-comprising stainless steels.
 4. The process according to claim 1, wherein the anode material is selected from a Cu-base alloy that comprises a4) 50 to 95% by weight of Cu and b4) 5 to 50% by weight of at least one further alloy component selected from Ag, Pb, Ni, or Zn.
 5. The process according to claim 4, wherein the base alloy is selected from nickel silver and cupronickel.
 6. The process according to claim 1, wherein the anode is a grid, expanded metal or metal plate.
 7. The process according to claim 1, wherein, in the electrolysis, as cathode an electrode comprising an electrode material is used, said electrode material selected from nickel, Ni-base alloys, Co-base alloys, Fe-base alloys, Cu-base alloys, silver, Ag-base alloys, RuO_(x)TiO_(x)-mixed oxides, platinized titanium, platinum, graphite, or carbon.
 8. The process according to claim 1, in which the electrolysis is carried out at a current density in a range from 1 to 100 mA/cm².
 9. The process according to any one of the preceding claims claim 1, in which the electrolysis is carried out at a temperature in a range from 10 to 100° C.
 10. The process according to claim 1, wherein the aqueous, alkaline lignin-comprising suspension or solution has a pH of at least
 10. 11. The process according to claim 1, wherein, as aqueous lignin-comprising suspension or solution, an aqueous lignin-comprising stream from paperstock, pulp ,or cellulose preparation is used.
 12. The process according to claim 1, wherein the aqueous, alkaline lignin-comprising suspension or solution is prepared by dissolving or suspending at least one lignin-comprising material in aqueous alkaline, wherein the lignin-comprising material is selected from lignin from black liquor, kraft lignin, lignin sulfonate, alkali lignin, organosolv lignin and corresponding residues from the paper industry, pulp, or cellulose preparation.
 13. The process according to claim 1, wherein the aqueous, alkaline lignin-comprising suspension or solution is selected from (a) an aqueous suspension or solution which is prepared by dissolving or suspending oxidized lignin in aqueous alkali, wherein oxidized lignin was obtained by oxidation of an aqueous, alkaline lignin-comprising suspension or solution, or (b) an aqueous suspension or solution which is prepared by depleting vanillin from an aqueous reaction mixture, wherein the reaction mixture was obtained by oxidation of an aqueous, alkaline lignin-comprising suspension or solution.
 15. The process according to claim 1, wherein the aqueous, lignin-comprising suspension or solution comprises 0.5 to 30% by weight of lignin or a derivative of lignin, based on the total weight of the aqueous, lignin-comprising suspension or solution.
 16. The process according to claim 1, wherein the vanillin formed in the electrolysis is continuously removed from the aqueous, lignin-comprising solution or suspension. 